The practice of applying a metal coating or veneer to various objects has long been practiced for practical and aesthetic purposes. For example, precious metals such as gold silver and platinum can be plated to the surfaces of inexpensive objects to give the objects the appearance, luster and physical properties of the precious metal. The application of gold “leaf” to diverse substrates has been used architecturally for centuries. Silver plating gives silverware the appearance and performance of solid silver without the expense. Coatings of less precious metals such as brass, nickel, bronze, chrome, tin, zinc and copper are also utilized for aesthetic and functional purposes.
A popular method of metal coating that has been around for more than a century is electroplating. Electroplating is a plating process that uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal. Electroplating is primarily used for depositing a layer of material to bestow a desired property (e.g., abrasion and wear resistance, corrosion protection, lubricity, aesthetic qualities, etc.) to a surface that otherwise lacks that property. Another application uses electroplating to build up the thickness of undersized parts. Electroplating has excellent transfer ratios but utilizes hazardous chemicals and processes. Electroplating is also limited to substrates which are electrically conductive and which can survive in the aquatic, acid environments necessary for the process.
Obtaining a uniform coating thickness with electroplating can be difficult for objects with multiple surfaces and complex shapes. The plating metal is preferentially attracted to external corners and protrusions, but unattracted to internal corners and recesses. These difficulties can be overcome with multiple anodes or a specially shaped anode that mimics the object geometry; however both of these solutions may increase cost.
In addition to the technical difficulty and sophistication required to practice electroplating the process is environmentally problematic. The stripping, etching, preparation and application steps associated with electroplating involve many hazardous chemicals such as sulfuric acid, chromic acid, cyanide and sodium hydroxide.
Another process for coating a metal surface is mechanical plating, also known as peen plating, mechanical deposition, and impact plating. Mechanical plating is a process that imparts a coating by cold welding fine metal particles to a work piece. The process typically uses fine metal powder, a tumbler on steel media to “peen” or pound the metal powder onto the target object's surface. This is used to plate screws, rivets, and the like.
Methods for depositing metal coatings such as chromium, nickel, cadmium, and copper in traditional electroplating processes have inherent pollution problems. Several alternative technologies exist to coat a substrate with metal without using electrolytic solutions or plating baths. These technologies do not eliminate the use of metal coatings, but they do eliminate the use of non-metal toxic components such as cyanide from the plating process. They also can reduce the amount of metal-contaminated wastewater and sludge that is generated from plating. These alternative technologies include thermal spray coating, vapor deposition, and chemical vapor deposition.
These alternative processes may have high unit-plating costs and, therefore, are typically used only for special applications where the cost of coating is not a major consideration. Another drawback to alternative metal deposition methods is that metal overspray or tailings from re-machining thick coatings from the alternative processes can actually increase waste generation.
Another conventional method for applying metal coatings involves spraying an object with a liquid polymer, such as unsaturated polyester resin, mixed with a catalyst and metal powder. This process is commercially called cold spray metal application. This process, like electroplating, is environmentally hazardous and wasteful of materials. U.S. Pat. No. 5,393,568 to Valente et al. describes a cold spray process that involves mixing three components: a reactive resin, a catalyst, and metal powder. The mixture is sprayed thru a high volume low pressure (HVLP) spray gun. The metal powder with its high specific gravity relative to the resin vehicle rapidly settles out of the mixture unless the mixture is continuously agitated, which is very difficult to do, if not impossible, during spraying. The mixture is also subject to premature catalization which can destroy the equipment used to apply it. At a minimum a significant amount of material is wasted as it must be prepared and used in batches and disposed of before it sets up in the equipment.
As with all spray processes, the transfer rate of the Valente process varies with the size and shape of the object to be coated and the skill of the operator. In practice, a large amount, often more than 50%, of the material mix may miss the target. This material is not recoverable and must be disposed of. The Valente process is also problematic in that the same settling which can occur in the equipment tends to take place on the sprayed object. The metal powder sinks to the bottom of the coating against the target object while the resin floats to the top before the resin can cure. In fact, polymerization may take 8 to 20 minutes, which may lead to running and sagging in addition to stratification. This creates a metal rich stratum against the object and a resin rich stratum away from the object. In order to achieve a decorative metallic surface, the resin must (after the mixture cures in place) be removed by abrasion to expose the metal sub-stratum. Such post finishing abrasive steps (sanding and polishing) can also limit the commercial viability of this process especially on complex shapes or highly detailed pieces. Unfortunately, attempts to overcome this by increasing the metal component make spraying the mixture problematic. Also, the Valente procedure includes the risk sagging and running, as with most liquid spray coatings.
There are several methods for imparting a metallic look to a substrate, all of which involve the use of reflective metallic particles or flakes. The application of metallic paints fall in this category. However, unlike true metalizing processes the use of metal flake in paint imparts only superficial surface properties. These materials typically cannot be post finished or polished as can true metals or true metal coatings. These materials achieve their metallic look by orientation of reflective flakes giving a synthetic metallic luster.
Powder coatings that are cured with heat can also achieve metallic looks with the use of metal flake, but, as with sprayable coatings, are difficult, if not impossible, to post finish as their surface is not truly metallic. Powder coatings are applied with a low pressure gun and use electrostatic charge for the initial adhesion to the target surface. Alternatively powder coatings may be applied by using fluidized bed technology which creates a cloud of powder coat particles into which the charged material is introduced.